Ok, first, the thumb...my daughter was exactly the same. By 5, she was sucking her thumb just when she was bored or tired, not usually in public, but still I thought it was time to stop. I told her that kids in school might make fun of her for it when she went to K, (she wasn't in Pre), but she wasn't too concerned. I continued my efforts to discourage it, including warning her about crooked teeth, and it took about 4 months or so into K before she finally came around. We had one last talk that night about how sometimes things that make us feel good are not good for us, and the examples of her brother who used to bite his nails and her dad who used to play video games too much. She got the point, struggled a little to fall asleep that night without the thumb, but was fine after that. Try not to stress too much about it, she'll probably grow out of it within the school year.
And you son...one suggestion, they have these hard plastic potty seats that fold up...their flat and you just unfold them and set them on top of a regular seat. They come in a large ziplock type bag. I used to carry one around on long days when my kids were little to cover the public toilet seats so I didn't have to try to lift them up, because they could never go like that. Maybe if you get one of those and even use it at home sometimes so he's used to it, then take it with you when you go out...it will be like it's "his" seat he gets to use everywhere. He could probably even take it to Preschool with him. Then public toilets might not seem so foreign to him, because he'll have his seat. It's possible too that sometimes in our efforts to explain to them that public seats are dirty and they can't just sit on them like normal, sometimes they develop a fear of them. Happened to my friend's daughter. So maybe the little folding seat would help. You can get them at Walmart, Bed Bath & Beyond, lots of places.